The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for treating subterranean formations, in particular, compositions and methods for cementing subterranean wells.
During the construction of subterranean wells, it is common, during and after drilling, to place a tubular body in the wellbore. The tubular body may comprise drillpipe, casing, liner, coiled tubing or combinations thereof. The purpose of the tubular body is to act as a conduit through which desirable fluids from the well may travel and be collected. The tubular body is normally secured in the well by a cement sheath. The cement sheath provides mechanical support and hydraulic isolation between the zones or layers that the well penetrates. The latter function is important because it prevents hydraulic communication between zones that may result in contamination. For example, the cement sheath blocks fluids from oil or gas zones from entering the water table and polluting drinking water. In addition, to optimize a well's production efficiency, it may be desirable to isolate, for example, a gas-producing zone from an oil-producing zone. The cement sheath achieves hydraulic isolation because of its low permeability. In addition, intimate bonding between the cement sheath and both the tubular body and borehole is necessary to prevent leaks.
Poor cement-sheath bonding may have several negative consequences. Interzonal hydraulic communication may interfere with proper well production, allow formation fluids to corrode the casing, and result in an environmental incident should hydrocarbons or saline fluids commingle with aquifers. The effectiveness of stimulation treatments may also be hampered, further limiting well production. Frequently, poor bonding is manifested by the presence of gaps, or “microannuli,” along the cement/casing interface, the cement/formation interface or both.
Cement systems that expand slightly (preferably less than about 1% linear expansion) after setting are a proven means for sealing microannuli and improving primary cementing results. The improved bonding is thought to be the result of mechanical resistance or tightening of the cement against the pipe and formation.
Some expansive cement systems rely upon the formation of the mineral ettringite to induce expansion. Ettringite is a calcium sulfoaluminate mineral that forms when the aluminate phases in Portland cement react with various forms of added calcium sulfate (usually calcium sulfate hemihydrate). Ettringite crystals have a larger bulk volume than the reactants from which they form; consequently, expansion occurs because of the internal pressure exerted upon crystallization. A limitation of ettringite-based systems is their inability to provide significant expansion at curing temperatures above about 76° C. (170° F.). Ettringite is not stable at higher temperatures and converts to another sulfoaluminate mineral that does not impart expansion.
Another type of expanding cements involves cement slurries containing high concentrations of NaCl, Na2SO4, or both. After the cement sets, cement expansion occurs because of internal pressure exerted by the crystallization of the salts within pores, and by chlorosilicate and chlorosulfoaluminate reactions. These systems may be effective at temperatures up to 204° C. (400° F.). However, the high cement-slurry salinity may cause casing corrosion, and may interfere with the performance of other cement additives—fluid-loss additives in particular.
Addition of calcined calcium oxide or magnesium oxide also may result in cement expansion after setting. The oxide hydration results in the formation of a hydroxide that is less dense than the reactants, thereby providing an expansive force within the cement matrix. These oxide systems have been employed successfully at temperatures up to about 260° C. (500° F.); however, the rate at which they react, and hence the expansion generated, may be difficult to control. If the additive hydrates too quickly (e.g., before the cement sets), little or no cement expansion may occur. If the additives hydrate too slowly, the expansion may occur too late and allow interzonal communication.
A more complete discussion of current expansive cement systems may be found in the following publication. Nelson E B, Drochon B, Michaux M and Griffin T J: “Special Cement Systems,” in Nelson E B and Guillot D. (eds.): Well Cementing (2nd Edition), Schlumberger, Houston (2006) 233-268.
Gas-generating agents may also be used to prepare expansive cements. When immersed in the high-pH aqueous environment, such as a Portland-cement slurry, the agents react and produce gas bubbles. For example, metals such as aluminum react in the presence of an aqueous, high-pH environment, resulting in the liberation of hydrogen gas. Without wishing to be bound by any theory, the pressurization effect is thought to cause the slurry to expand against the boundaries into which it has been injected—for example, the annular region between the casing string and the borehole wall. Such expansion may improve bonding at the casing/cement and casing/borehole-wall interfaces.
Gas-generating agents also have utility for preventing annular-fluid migration. In this context, cements containing gas-generating agents are commonly referred to as “compressible cements.” Without wishing to be bound by any theory, such cements are thought to maintain the cement-pore pressure above the formation-pore pressure, thereby preventing ingress of gas or other formation fluids into the cemented annulus. A more complete discussion of gas-generating agents for preventing annular-fluid migration may be found in the following publication. Stiles D: “Annular Formation Fluid Migration,” in Nelson E B and Guillot D. (eds.): Well Cementing (2nd Edition), Schlumberger, Houston (2006) 289-317.
One difficulty with many gas-generating agents is their high reactivity when exposed to water. In the context of well cementing, the optimal period for gas generation is between the time at which the cement slurry enters the annulus and the time at which the slurry sets and begins developing strength. Premature gas generation may result in the release of gas at the surface, thereby reducing the ultimate slurry compressibility, and possibly presenting a safety hazard. Gas generation after the slurry sets may also reduce the ultimate slurry because of mechanical resistance exerted by the strengthening cement matrix.
Despite the valuable contributions of the prior art, it would be advantageous to delay the reactions between gas-generating agents and water, and maximize their intended expansion and pressurization effects.